The importance of achieving accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such alignment difficult both to achieve and to maintain. For example, one problem in the construction of laser transmitters is that of efficiently coupling the optical output from a laser diode into an optical fiber. To obtain efficient coupling, the fiber end is desirably precisely aligned with the emitting area of the laser. When such alignment is achieved, the fiber is then fixed in place, ideally by a method that ensures alignment is sustained throughout the device lifetime.
Typically, a fiber is held in alignment with a laser using either epoxy, laser weld, or solder attachment techniques. Epoxy attachment is low cost but is not reliable over a long period of time due to outgassing and alignment shifts arising from aging and temperature cycling. Laser weld techniques are reliable but require costly ferrulization of the fiber and specially designed mounts or clips to allow weld attachment of the ferrulized fiber. Solder attachment techniques, on the other hand, are reliable and low cost, and thus have become prevalent in the art.
Currently, solder attachment may be attained through many techniques including: the application of a wire solder melted as it is fed into a desired location to secure a fiber; a sheet based preform that is placed either underneath, on top of, or surrounding the optical assembly prior to soldering; the application of molten solder directly at the point of attachment; and a blob preform with a fiber feedthrough through which a fiber is threaded prior to soldering.
The current methods of solder attachment undesirably impede motion of the optical fiber due to the weight of the preform placed over the fiber. Consequently, it may be more difficult to align and optimize a laser output signal to an optical fiber when both the fiber and the solder preform must be moved. Additionally, a desirable alignment of the fiber with the laser may be altered during soldering due to, for example, capillary stresses from the molten solder. These stresses may result in a misalignment of the fiber upon attachment even though the fiber was aligned before the solder was melted.
Even further problems may arise from the step of soldering a wire or preform to an optical fiber, which may include resistive heating, laser heating, or oven heating to melt the wire or preform. In this step, the optical fiber may be subjected to high power laser radiation and/or high temperatures needed to melt the solder, resulting in undesirable stresses due to thermal shock as well as possible alterations of optical properties of the optical fiber.
In the case of fiber feedthrough solder preforms, an extra manufacturing step may be required to feed an optical fiber through the solder perform, adding undesirable manufacturing costs.